1. Field of the Invention
The invention relates to a system having a coextrusion adapter for the production of coextruded webs or extrudates from two or more plastics that can be processed thermoplastically. The coextrusion adapter brings together the streams of molding compounds being conveyed from two or more extruders into one common extrudate with two or more layers and conveys this extrudate into a slit die in which it is stretched to the desired width and correspondingly reduced in thickness. The average thickness of the layers involved thus decreases in the same proportion as the total thickness.
2. Background of the Related Art
There is known from DE-A 35 05 257 a coextrusion adapter designated as a collector block and forming unit. In this coextrusion adapter a thicker main extrudate and a thinner secondary extrudate are brought together. The combined extrudates are subsequently extruded via a slit die to form a multilayered slab or sheet which contains a covering layer of the material from the secondary extrudate.
As a rule the covering layer is not completely uniformly distributed over the width of the extruded web. This can have a series of causes, for example unevenness in the cross-section forms of the main and secondary extrudate, uneven temperatures within the main extrudate or the secondary extrudate from which varying melting viscosities and a correspondingly varying flow behavior follow, or varying flow resistances of the molding compound extrudates within the coextrusion adapter or the extrusion die. It is difficult to identify the causes of the uneven distribution of thickness, and even more difficult to eliminate them.
According to DE-A 34 05 257 the thickness and the uniformity of the coextruded layer are controlled by regulating the thickness profile of the secondary extrudate in the coextrusion adapter by separate control means. For this purpose a slide that can be adjusted by adjusting screws is used which protrudes more or less deeply into the secondary runner in which the secondary extrudate flows in the direction of the juncture of both extrudates. By operating the adjusting screws that act on the slide, the flow resistance in the secondary extrudate can be influenced, and thus also its thickness in relation to the main extrudate.
Such adjustment is however not suitable for evening out unevennesses in the distribution of the layer thicknesses over the width of the extruded web, for example if the covering layer in the middle is thicker or thinner than that on the edges or has some other nonlinear profile. In this case a slide is used in the coextrusion adapter, whose profile is configured opposite to the profile of the covering layer; i.e., the slide protrudes deeper into the melt flow as the profile of the covering layer of the extruded web becomes thicker, and vice versa. The slide profile can only be adapted empirically to the unevenness of the covering layer profile, because it is not possible to assign a specific point of the slide profile to a specific area of the covering layer profile. Furthermore, it cannot be predicted how sharply the slide profile must be changed to effectively even out an unevenness in the covering layer profile detected in a crosswise direction.
To adapt the slide profile, the extrusion must be repeatedly interrupted, the coextrusion adapter cooled and opened, the slide taken out and mechanically reworked and reinstalled, and the extrusion started up again. If the change made proves to be insufficient, the adaptation process must be repeated until the necessary uniformity of the covering layer profile is achieved. Even then a further adaptation can become necessary if after prolonged operation the flow conditions in the extrusion unit have changed and new unevennesses in the covering layer occur.
To simplify the adaptation of the slide, R. Reitemeyer ("Kunststoffe" [Plastics] Volume 78, 1988, No 5, pages 395-397) proposed configuring the coextrusion adapter so that the slide can be pushed in laterally like a cassette into a corresponding guide. In this way a complete dismantling of the adapter at every adaptation step is avoided, but not the interruption of the coextrusion. Further, he proposed making the slide in several parts, thus to apparently be able to combine the sections that protrude at varying depths into the melt flow.
In an extrusion device according to DE-A 37 41 793 this configuration was further developed in that the sections of the multipart slide protruding in the crosswise direction next to one another into the secondary runner are placed so that they can be shifted in the adjustment direction while pressed against one another and are adjustable by separate control means. The adjustment of each of the sections is sensed by a measuring device and the measurement is used to influence the control means. For this purpose piezotranslators can be used. The recorded values, relative to the special plastic web which is being produced at the time, can be fed to a computer and incorporated into a program.
Thus it is certainly possible to adjust the extrusion unit quickly and largely automatically to different, previously determined extrusion conditions. On the other hand, the known device does not make it possible to adapt the extrusion device automatically to changing extrusion conditions during operation. Such changes can occur from multiple causes: for example because of temperature fluctuations which can result from movements of air in the surroundings of the extrusion unit, from changes in the melt viscosity as a result of a slightly different nature of the plastic that is fed in, or from changes in the flow resistance of the melt flow as a result of wear or corrosion in the flow paths. In such cases the melt flow in the coextrusion adapter can be adapted to the changed conditions by acting on the control elements, or the adjustment program can be correspondingly changed. However, such an adjustment presupposes conscious interventions by the operating personnel, which can take place only with considerable hesitation.